The Group III nitride semiconductors (abbreviated in this invention as “nitride semiconductors”) possess a band gap of a direct transition type of energy corresponding to the visible light through the ultraviolet light range and permit highly efficient light emission and have been consequently reduced to commercial products, such as light-emitting diodes (LEDs) and laser diodes (LDs). Particularly the realization of white light-emitting diodes due to their combination with fluorescent materials has been expected as a new field for the application of light-emitting diodes.
The output of a light-emitting diode is greatly affected by the efficiency of light extraction from a relevant device to the exterior. On this efficiency of light extraction, the total reflection on the surface of the device functions as a factor for imparting a great effect. It is known very well that when a light advances from a semiconductor layer having a large refractive index toward the exterior having a small refractive index, the part of the light exceeding the critical angle (θc) undergoes total reflection on the phase boundary of the device and the light is not extracted to the exterior of the small refractive index.
As a means to avoid the restraint imposed on the light extraction by the total reflection on the phase boundary, a method for coarsening the phase boundary (refer, for example, to Japanese Patent No. 2836687) and a method for forming the shape of the device to utilize the Escape Cone of another surface (refer, for example, to Japanese Patent No. 2784537) have been known.
Ordinary light-emitting diodes are mostly shaped in the form of a rectangular hexahedron on account of the ease of manufacture of device. In this case, the number of phase boundaries that allow formation of Escape Cone is limited to six.
The magnitudes of the particular solid angles of Escape Cones are decided in conformity with the characteristics of the individual boundary phases. As a matter of course, the total of the magnitudes of the individual solid angles increases proportionately to the number of the phase boundaries.
The output of the light-emitting diode device, therefore, is increased by so shaping the light-emitting diode device as to increase the number of planes from the ordinary rectangular hexahedron. The ultimate in the exaltation of the output by the increase of the number of planes resides in the method for forming the device in the shape of a hemispherical dome. Numerous pieces of prior art have been already introduced to this ultimate method (refer, for example, JP-A SHO 61-222285).
The method which resorts to the increase in the number of planes of the device, however, allows no easy formation of the device and consequently entails the problem of seriously degrading the yield of product. Thus, a method which comprises imparting fine grooves to the surface of the device by an etching operation, thereby making substantial addition to the number of planes has been proposed (refer, for example, to JP-A 2002-164574 and JP-A 2004-87930). JP-A 2004-87930 discloses the role of current-preventing grooves which are so formed as to prevent the electric current from being concentrated directly under an electrode.
When the number of planes is substantially increased for the purpose of enhancing the light-extraction efficiency, increasing the surface areas of the consequently added planes increases the effect monotonously. When the surface of the device is subjected to groove formation, the deeper the formed grooves, the higher the light-extraction effect is.
The nitride semiconductor, however, is deficient in reactivity and is not easy to process. Generally, it is processed predominantly by the dry etching which uses a chlorine type gas and performs the etching with a reaction species resulting from exciting the etching gas with plasma. Despite the adoption of the dry etching, the processing speed is prominently low and barely reaches several μm/h.
Further, since the process resorts to the plasma of high energy, it is known to entail electrical breakdown by charged particles and deterioration by ultraviolet radiation. When this etching is continued for a long time with a view to enlarging the amount of processing, it brings no negligible influence and poses the problem of preventing the light-emitting diode from acquiring any enhanced characteristic properties.
This invention is aimed at enhancing the light-extraction efficiency of a nitride semiconductor light-emitting device. It is, therefore, aimed at facilitating the process of shaping the device and enhancing the yield thereof.
This invention has been perfected for the purpose of accomplishing the objects mentioned above and embraces the following aspects of invention.